An OFDM method has been used in digital terrestrial broadcasting known as DVB-T (Digital Video Broadcasting Terrestrial) in Europe. The OFDM method is also used in digital terrestrial broadcasting known as ISDB-T (Integrated Services Digital Broadcasting Terrestrial) in Japan.
The OFDM method is one of multicarrier modulation methods. In the OFDM method, digital information is transmitted by modulating on a symbol by symbol basis a multiplicity of carriers that are orthogonal in frequency to each other. In a synchronous detection segment, an SP (Scattered Pilot) signal having a known amplitude and phase is placed every 12 carriers within a symbol. The SP signal is shifted by three carriers in each symbol in a frequency axis direction and placed in every four symbol periods in a time axis direction. A differential detection segment does not include an SP signal (see Patent document 1).
In general, an OFDM demodulator equalizes a received signal by extracting an SP signal from a synchronous detection segment, estimating transmission path characteristics by interpolating the extracted SP signal in a frequency axis direction and a time axis direction, and dividing an information transmission signal by the estimated transmission path characteristics (see Patent document 1).
Transmission path characteristics vary significantly in a mobile reception environment such as cellular phones and car navigation systems. In a known technology, estimation accuracy of transmission path characteristics is improved as follows: a received signal is demodulated by using transmission path characteristics estimated by interpolating an SP signal. Transmission path characteristics are then obtained again by using a hard decision result. More reliable ones of these transmission path characteristics are used (see Patent document 2).
Patent document 1: International Publication No. WO99/01956 pamphlet
Patent document 2: Japanese Patent Laid-Open Publication No. 2004-96703